Transformerless power supply with line to load isolation

ABSTRACT

A transformerless power supply is disclosed in which a source capacitor has its charge maintained at a fraction of the AC supply voltage and a plurality of sink capacitors are sequentially switched between a charging connection to the source capacitor and a discharging connection to a load circuit. The load circuit is isolated from the line at all times to maintain safety protection. The use of a comparatively high switching speed will reduce the filter requirements in the load circuit and this together with the elimination of the transformer results in very substantial size and weight reductions from conventional supplies. Voltage regulation at the load is achieved by controlling the voltage to which the sink capacitors are charged from the source capacitor.

United States Patent [72] Inventors Jack A. Dickerson Raleigh, N.C.;Gerald R. Ottaway. Pleasant Valley, N.Y. [21] Appl. No. 887,023 [22]Filed Dec. 22.1969 [45] Patented Aug. 3, 1971 [73] AssigneeInternational Business Machines Corporation Armonk, N.Y.

[54] TRANSFORMERLESS POWER SUPPLY WITH LINE T0 LOAD ISOLATION 7 Claims,7 Drawing Figs.

[52] [1.5. CI 321/43, 320/1, 321/47 [51] lnt.Cl 02m 7/12 [50] Field ofSearch 321/4, 7, 43, 47; 320/1 [56] References Cited UNITED STATESPATENTS 3,247,444 4/1966 Clarke et al. 321/4 oz-ozmcornm T0 SIlTCll T0SIllCll C VOLTAGE TECHNICAL DISCLOSURE BULLETIN, T. J. Harrison & J.Jursik, Vol. 6, No. 8 Jan. 1964, page 25.

Primary Examiner-William M. Shoop, Jr.

AflomeysHanifin and Jancin and Delbert C. Thomas ABSTRACT: Atransformerless power supply is disclosed in which a source capacitorhas its charge maintained at a fraction of the AC supply voltage and aplurality of sink capacitors are sequentially switched between acharging connection to the source capacitor and a discharging connectionto a load circuit. The load circuit is isolated from the line at alltimes to maintain safety protection. The use of a comparatively highswitching speed will reduce the filter requirements in the load circuitand this together with the elimination of the transformer results invery substantial size and weight reductions from conventional supplies.

Voltage regulation at the load is achieved by controlling the voltage towhich the sink capacitors are charged from the source capacitor.

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"SINK'CAPACITOR 13 T0 LOAD 17 DISCHARGE 'SINK'CAPACITOR 13 T0 SOURCE 11i CHARGE FIG. 5

V REF PATENTEDAUI; 3:972 3,596,368

SHEET 3 [If 3 FIG. 6

SWITC COMPARATOR V REF FIG. 7

l NPUT FROM SWITCHING F LOGIC SWITCHEO CONTACTS TRANSI OIRMERLESS POWERSUPPLY WITH LINE T LOAD ISOLATION OBJECTS OF THE INVENTION The inventionset out herein relates to transformerless power supplies havingisolation between the load circuit and the supply circuit, and moreparticularly to such power supplies tor providing a low voltage, highcurrent supply from a commercial powerline.

Transformerless power supplies are well known in versions such as areused in the older AC-DC radios. In that type of power supply, a singlediode rectifies the power supply voltage for a half wave supply or apair of oppositely poled diodes individually charge series-connectedcapacitors in a peak voltage doubling circuit. A full-wave bridgerectifier can also be used. In each of these supplies, the outputvoltage at no load is the line voltage peak or double that figure andthe load voltage remains high even under normal loads, such powersupplies do not meet safety requirements for isolation between the linecircuit and the load circuits of commercial machines. In most suchsupplies, isolation is only that provided by the back resistance of adiode or through a large capacitor and a failure can permit dangerousvoltages to be applied to the load side.

In commercial business machines such as data processing machines, poweris required at large currents but low voltages. The isolationspecifications for line to load insulation have usually been met byusing transformers to convert the line voltage to a low output voltagebefore rectification and filtering. This type of power supply needsheavy transformer secondary and filter choke windings with massive ironcores.

The present invention provides a commercial-type low voltage powersupply which docs not use line transformers or large filter componentsand consequently can provide a size reduction of about 60 percent to 70percent as compared with conventional supplies and shows a weightreduction to around percent to percent of the usual power supplies. Thecost reduction is also substantial since solid-state components andswitches can be used, and these will have substantial advantages overmechanical devices. In this new power supply, the AC supply is rectifiedeither full wave or half wave starting at a phase angle near the end ofthe decreasing voltage part of the cycle to charge a source capacitor toa voltage somewhat higher than the desired load voltage. A plurality ofsmaller sink capacitors are then sequentially connected to the source tobe charged to the load voltage and are then sequentially switched overto the load circuit to power the load. A high switching speed generatesonly a small high frequency ripple in the load circuit which can beeasily filtered by use of small components. Any powerline frequenciescan be eliminated from the load circuit by regulating the voltage towhich the sink capacitors are charged from the source capacitor.

It is a primary object of this invention to reduce the size and weightof low voltage, high current power supplies as used in commercialapplications by eliminating the conventional transformer andsubstantially reducing the filter components normally required inpowerline conversion systems.

It is another object to provide such a power supply wherein the loadcircuit is isolated from the line circuit by a substantial impedancecorresponding to that between the windings of a transformer.

Still another object is to provide a transformerless power supplywherein the output current is supplied with a high ripple rate enablingeffective filtering with the use of smaller filter components.

A further object is to reduce the size and weight of a power supply bythe use of a number of capacitors which are sequentially switchedbetween a DC voltage source and a load circuit to provide both source toload isolation and a high frequency ripple voltage in the load circuit.

A still further object is to provide voltage-regulating systems for sucha power supply to maintain a load independent output voltage, eliminateany load circuit ripple at powerline frequencies and to improve overallcircuit conversion efficiency.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention. as illustrated inthe accompanying drawings.

DRAWINGS In the accompanying drawings:

FIG. 1 is a diagrammatic showing of the improved power supply includingone type of voltage-regulating connection.

FIG. 2 shows one type of preregulator for the power supply.

FIG. 3 shows a modification of the preregulator of FIG. 2 to provide forimproved regulation of the source voltage.

FIG. 4 is a timing chart showing the time relationships between theoutputs of the sequencing switches.

FlG. 5 is one type of output voltage regulator in which each sinkcapacitor has its charge independently monitored.

FIG. 6 is another type of output voltage regulator in which the outputvoltage is monitored to control connection of the sink capacitors to thesource.

FIG. 7 is an example of one type of isolated switch which can be used inthe power supply.

DETAILED DESCRIPTION The preferred embodiment of our invention, asdiagrammed in FIG. 1, provides a preregulator 10 which rectifies theusual v. AC line voltage to a lower DC (pulsating) level. Preregulator10 will be later set out in more detail, but broadly serves to selectthe point on the decreasing part of the line voltage at which the linewill be connected to charge a source capacitor 11. The maximum voltageto which capacitor 11 will be charged on any powerline cycle will becontrolled by preregulator 10 determining the time and duration ofconnection of capacitor 11 to the powerline. A plurality of sinkcapacitors 13A, 138, etc. four such capacitors being indicated in FIG. 1for purposes of illustration, are connectable to source capacitor 11 bya double pole switching circuit 14A, 143, etc. Capacitors 13 are alsoconnectable by double pole switches 15A, 1513, etc. to a load circuit 16to supply power to a load 17. The switches 14 and 15 are controlled intheir operation by a sequencing switch 20 which will connect one sinkcapacitor 13 to source capacitor 11 for charging and another sinkcapacitor 13 to loadline 16 are discharge through load 17. Switches 14are also controlled by a voltage regulator 21 which limits the voltageto which capacitors 13 are charged from source capacitor 1 1.

More specifically, one embodiment of preregulator 10 is shown in FIG. 2.An AC power source or preferably a fullwave-rectified AC source isconnected to terminals 25. A capacitor 26 across the terminals and asmall inductance 27 in series with the upper terminal are a line filterto minimize transmission of switching transients from the preregulatorto the powerline. The other end of inductor 27 is connected to the anodeof a silicon-controlled rectifier (SCR) 28 which has its cathodeconnected to one side of source capacitor 11. The lower terminal 25 isconnected directly to the other lead of capacitor 13. The gate voltageon SCR 28 is controlled to set the firing point of SCR 28 with referenceto the AC cycle phase angle and to thereby determine the maximum voltageto which capacitor 11 will be charged. The gate control of SCR 28 isthrough a rectifier 30 having its anode connected to the right end ofinductor 27 and its cathode connected through a resistor 31 to thecathode of a zener diode 32 having its anode connected to the lowerterminal 25. The cathode voltage on zener diode 32 will thus follow theinput voltage until the ,zener diode breaks down, will be substantiallyconstant until the zener diode breaks down, will be substantiallyconstant until the input voltage decreases to the zener rating and willthen again follow the input. A diode 33, resistor 34 and capacitor 35 inseries across zener diode 32 provide a cyclicallydelayed voltage at thejunction of resistor 34 and capacitor 35. This delayed voltage isapplied to the anode of a programmable unijunction transistor 36 whosegate is connected to the cathode of zener diode 32. Transistor 36 issimilar to an SCR but has the characteristic that initial conductionwill be prevented until the anode voltage is about one-half volt higherthan the gate voltage. One unijunction transistor of this type isproduced by General Electric Company and is marketed as Type D 13 T I.

In the circuit shown, it will be evident that with a rising inputvoltage, the anode voltage will lag behind that of the gate and thisrelation will be maintained until near the end of the cycle of thevoltage on terminals 25, the input voltage decreases below the breakdownvoltage of zener diode 32. At this point the voltage of the zenercathode and that on the gate of transistor 36 will start to decrease andwill shortly go below that on the anode which will be maintained bycapacitor 35. When the gate to anode voltage reaches the firingpotential, unijunction transistor 36 will conduct and dischargecapacitor 35 through resistor 37 connected to its cathode. Thisdischarge produces a sharp pulse on the cathode circuit and the pulse ispassed through a capacitor 38 to the gate circuit of SCR 28 to fire SCR28 and recharge capacitor 11. This sequence of operations will occur oneach cycle of the input voltage and will serve to maintain capacitor 11charged to a voltage determined by the rating of zener diode 32.

Since the minimum voltage to which source capacitor 11 dischargesbetween the cycles of the line voltage depends upon the load power beingdrawn, an additional circuit shown in FIG. 3 may be installed to improvepreregulation. This preregulation will maintain approximately uniformthe voltage to which the source capacitor 11 is charged under all loadcurrent conditions and will substantially improve circuit efficiency. InFIG. 3, the circuit from input terminals 25 to zener diode 32 and thegating circuit for SCR 28 from diode 33 are similar to those of FIG. 2but a pair of resistors 41 and 42 in series are placed across zenerdiode 32 with the anode of diode 33 connected to their junction. Thelower resistor 42 is shunted by a photosensitive resistor 43. Alight-emitting diode 44 is connected across source capacitor 11 and isoptically coupled to photoresistor 43. The optically coupledphotoresistor combination is commercially available from MonsantoCompany as their item MCR I.

In operation, resistors 41, 42 and 43 act as a voltage divider to applya part of the zener diode voltage to the trigger circuit for SCR 28. Theportion of the voltage applied will depend upon the resistance ofresistor 43 and this in turn will be determined by the voltage on sourcecapacitor 11 rises, diode 44 generates more light to decrease theresistance of resistor 43 and this reduces the voltage fed to thetrigger circuit for SCR 28. This reduction in trigger circuit will tendto retard the firing point of SCR 28 to reduce the voltage switched toSCR 28. When the output voltage decreases, the opposite action will beinitiated. This type of feedback control can be so set that the averagevoltage of capacitor 11 is substantially independent of the load currentdrawn from the power supply.

The switches 13 and 14 of FIG. ll may be any conventional type ofcurrent switching circuit such as a power transistor having its basecontrolled for the switching function. Generally, however, such switchesdo not meet commercial standards for isolation since under some failureconditions, the line and load circuitry could be connected. It is,therefore, preferred to use an isolated switch in which the controlcircuit is separated from the switched circuit. Light-actuated switchesin which the switch control is by emitted light are known but are notsufficiently developed to switch currents of the magnitude and voltageneeded in commercial applications.

The isolated switch shown in FIG. 7 is one type which has been foundsatisfactory in the present power supply. This switch has a powertransistor 50 and a diode 49 in the switched circuit for the main switchwith a zener diode 51 across transistor 50 to bypass any damagingvoltage surges and a base control circuit to control current flow. Thebase control circuit comprises the secondary 52 of a small transformerwith a diode 53 and capacitor 54 across the secondary 52. The base oftransistor 50 is connected to the junction of diode 53 and capacitor 54and has a resistor 55 connected between it and the transistors emitteras a capacitor discharge circuit. The emitter of transistor 50 isconnected to the other lead of capacitor 54. In this circuit an ACvoltage in secondary 52 will be rectified and the resulting DC voltageapplied to the base of transistor 50 to establish current flow throughtransistor 50. When the AC voltage is removed, capacitor 54 dischargesthrough resistor and current flow is terminated. The AC voltage insecondary 52 is generated by an intermittent current through transformerprimary 57. This intermittent current is derived from a switchingvoltage by a conventional unijunction circuit. The unijunction circuitcomprises a resistor 58 from a positive logic voltage to the anode ofthe unijunction 59 with the unijunction cathode connected throughprimary 57 to the negative logic voltage. A resistor 60 and capacitor 61are connected in series from the positive to the negative logic voltagesand their junction is connected to the gate of unijunction 59. With sucha connection, the unijunction 59 will start to conduct when the gatevoltage rises to a given point, will conduct while the gate voltage isdecreasing due to discharge of capacitor 61, and will stop conductingwhen the gate voltage reaches a lower limit. The capacitor 61 will thenstart to recharge until its voltage is high enough to start theunijunction conducting to repeat the cycle.

The switches 14 and 15 are controlled to sequentially connect the sinkcapacitors 13 to source capacitor 11 and to 211' ternately connect themto the load circuit 16 so that one sink capacitor 13 is connected to theload at all times. The switching sequence is controlled by thesequencing switch 20 of FIG. 1 which cycles to close a switch 14 tocharge a capacitor 13 from the source capacitor 11 during a firstinterval, opens switch 14 during a second interval, closes switch 15 todischarge the capacitor 13 through the load 17 during a third intervaland opens the switch 15 during a fourth interval. The cycles for thesink capacitors 13A, 13B, 13C, etc. are staggered so that only onecapacitor is connected to the load at a time.

It has been found that the smoothest and easiest filteredoutput voltageis obtained with the highest switching cycle speed. At higher speeds,however, the large components needed to carry the heavy current do notswitch cleanly and a compromise speed must be selected. It has beenfound that a switching speed of l to 5,000 cycles per second is easilyobtainable and gives a satisfactory power output. FIG. 4 indicates theoutputs of the switch 20, and it may be seen that a cycle is dividedinto eight parts (twice the number of sink capacitors). Each capacitor13 has its switch 15 closed during two periods, both switches 14 and 15for a capacitor are open for a third, switch 14 is then closed for fourperiods and then both switches are open for the last period. With a twoperiod offset between the cycles for the sink capacitors 13A, 13B, etc.one capacitor will always be discharging into load circuit 16. Thefour-period length for closure of switch 14 may not always be necessarybut has been included in some embodiments to insure adequate charging ofa sink capacitor 13. The sequencing switch 20 may be a conventionaloscillator-controlled ring circuit of the type well known for timingpurposes in data processing machines.

Several systems for regulation of the output voltage on circuit 16 areavailable and the most satisfactory ones control the charged voltage oncapacitors 13. In FIG. 5, the voltage to which a sink capacitor 13 ischarged is individually controlled. As shown, switch 15A is controlleddirectly by the T1 output from sequencing switch 20 but switch 14A iscontrolled through an AND circuit 67 from both the T5 output of switch20 and an output 65 ofa voltage comparator 66 which may be of anisolated input to output type using balanced oscillators driven by theload voltage and a reference voltage respectively. The voltage oncapacitor 13 is continuously compared with a reference voltage on a lead68 and when the capacitor voltage is lower than the reference voltage asit presumably would be after it has discharged to a load, the output ofcomparator 66 on lead 65 is at a high level. Now when sequencing signalT5 is present, AND 67 has a logic output signal to close switch l4 andcharge capacitor 13. The voltage on capacitor 13 will continue to bemonitored and when it reaches that of the voltage reference, comparator66 will drop the voltage of its output 65 to open switch 14 and stopcharging of capacitor l3. A similar circuit is provided for each sinkcapacitor 13 to control its charging voltage.

This regulation circuit may be made less complicated by using a singlecomparator 66 to control all switches 14. In this type of circuit, theoutput of the one comparator 66 goes to all ANDs 67 and its inputs aregated to the respective capacitors 13 in turn through gates controlledby their timing signal for their switches 14. With this regulatingcircuit, the charging periods should not overlap and a four-part timingperiod is sufficient.

Another type of voltage regulator as shown in F IG. 6 can be used if thesink capacitors 13 are sufficiently large to change in voltage onlyslightly during one timing cycle and if their charging rate can be sorestricted as to supply in each cycle only a little more charge than themaximum charge delivered by a capacitor 13 to the load circuit 16. Inthis circuit, isolated comparator 66 has its input connected to the loadcircuit 16 and its output 65 controls all of the AND circuits 67. Thisis effectively an ON-OFF type of regulator in which the sink capacitors13 are recharged only when the output voltage is below the referencevoltage. Since there is a delay of at least a part of a switching cyclebetween detection of an output voltage variation and the start of thecorrection for the variation, this type of ON-OFF regulation has aninherent ripple whose frequency is dependent upon comparator responsetime. It is, however, comparatively inexpensive and satisfactory wheresome power supply ripple is permissible.

This type of ON-OFF voltage regulation has a further advantage in thatwhen the load 17 is at a substantial distance, it can compensate forvoltage drop in the power leads 16. There is little current used by thecomparator 66 and if its input leads for the voltage to be regulated areconnected to lines 16 at the load end, the load voltage will be comparedwith the reference voltage and the output voltage at switches will varyin accordance with the load current,

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formand details may be made therein without departing from the spirit andscope of the invention.

What we claim is:

l. A direct current power supply comprising a source capacitor, aphase-controlled rectifier for charging said source capacitor to apredetermined voltage from an AC power source having an RMS voltagesubstantially higher than said predetermined voltage, a plurality ofsink capacitors, a first switch means for each sink capacitor to connectsaid sink capacitor across said source capacitor, a second switch meansfor each sink capacitor to connect said sink capacitor to a loadcircuit, and a sequencing switch to alternately operate said first andsecond switch means to enable each sink capacitor to transfer a chargefrom said source capacitor to said load circuit, said sequencing switchcontrolling said first switch means for said plurality of switchcapacitors to operate in a fixed sequence and controlling said secondswitch means to operate in the same sequence with a time displacement.

2. A power supply as set out in claim 1 including a loadvoltage-regulating control, said control comprising for each sinkcapacitor, a comparing circuit to provide a signal voltage when thevoltage of its sink capacitor is greater than a reference voltage and aswitch control circuit to prevent operation of said first switch meanswhen said signal voltage is present.

3. The power supply set out in claim 1 including an outputvoltage-regulating circuit responsive to the output voltage of saidpower supply to prevent closure of any of said first switch means solong as said output voltage is greater than a predetermined voltage.

4. The power supply set forth in claim 1 including an outputvoltage-regulating circuit comprising a voltage comparator to provide asignal voltage when the voltage applied to its input circuit is above areference level, a control means for said first switch means to open anyof said first switch means when said signal voltage is present, and acircuit connecting one of said sink capacitors at a time to the inputcircuit of said comparator.

5. A direct current power supply comprising a source capacitor, an ACpower input, a controlled-phase rectifier between said power input andsaid source capacitor, a phase control network for said rectifier tocharge said source capacitor to a voltage substantially lower than theRMS voltage of said AC power input, a plurality of first switchesconnected to said source capacitor, a sequencing switch to sequentiallyclose and sequentially open said plurality of first switches, a sinkcapacitor for each first switch, each said sink capacitor beingconnected for charging to said source capacitorby its said first switch,a load circuit, a plurality of second switches, each to connect one ofsaid sink capacitors to supply power to said load circuit, connectionsfrom said second switches to said sequencing switch to open and closesaid second switches in the same sequence as said first switches butwith a predetermined time delay and output voltage-regulating means toadditionally control the closing and opening of said first switches. a i

6. A power supply as set out in claim 5 wherein said voltageregulatingmeans comprises a voltage comparator for each sink capacitor and a firstswitch control means to prevent connection of its associated sinkcapacitor to said source capacitor so long as the voltage of said sinkcapacitor exceeds a predetermined level.

7.'A low voltage power supply comprising a source capacitor, an AC powerinput, a phase-controlled rectifier to connect said AC input to saidsource capacitor during the decreasing voltage part of the cycle of saidAC power to charge said source capacitor to a relatively small part ofthe RMS voltage of said power input, a plurality of sink capacitors,first sequentially operated switches to connect said sink capacitors tosaid source capacitor seriatim, a second set of switches operated in thesame sequence as said sequentially operated switches but having apredetermined time delay therefrom to connect said sink switches to anoutput load cir cuit, and output voltage control means to controlopening of said first sequentially operated switches.

1. A direct current power supply comprising a source capacitor, aphase-controlled rectifier for charging said source capacitor to apredetermined voltage from an AC power source having an RMS voltagesubstantially higher than said predetermined voltage, a plurality ofsink capacitors, a first switch means for each sink capacitor to connectsaid sink capacitor across said source capacitor, a second switch meansfor each sink capacitor to connect said sink capacitor to a loadcircuit, and a sequencing switch to alternately operate said first andsecond switch means to enable each sink capacitor to transfer a chargefrom said source capacitor to said load circuit, said sequencing switchcontrolling said first switch means for said plurality of switchcapacitors to operate in a fixed sequence and controlling said secondswitch means to operate in the same sequence with a time displacement.2. A power supply as set out in claim 1 including a loadvoltage-regulating control, said control comprising for each sinkcapacitor, a comparing circuit to provide a signal voltage when thevoltage of its sink capacitor is greater than a reference voltage and aswitch control circuit to prevent operation of said first switch meanswhen said signal voltage is present.
 3. The power supply set out inclaim 1 including an output voltage-regulating circuit responsive to theoutput voltage of said power supply to prevent closure of any of saidfirst switch means so long as said output voltage is greater than apredetermined voltage.
 4. The power supply set forth in claim 1including an output voltage-regulating circuit comprising a voltagecomparator to provide a signal voltage when the voltage applied to itsinput circuit is above a reference level, a control means for said firstswitch means to open any of said first switch means when said signalvoltage is present, and a circuit connecting one of said sink capacitorsat a time to the input circuit of said comparator.
 5. A direct currentpower supply comprising a source capacitor, an AC power input, acontrolled-phase rectifier between said power input and said sourcecapacitor, a phase control network for said rectifier to charge saidsource capacitor to a voltage substantially lower than the RMS voltageof said AC power input, a plurality of first switches connected to saidsource capacitor, a sequencing switch to sequentially close andsequentially open said plurality of first switches, a sink capacitor foreach first switch, each said sink capacitor being connected for chargingto said source capacitor by its said first switch, a load circuit, aplurality of second switches, each to connect one of said sinkcapacitors to supply power to said load circuit, connections from saidsecond Switches to said sequencing switch to open and close said secondswitches in the same sequence as said first switches but with apredetermined time delay and output voltage-regulating means toadditionally control the closing and opening of said first switches. 6.A power supply as set out in claim 5 wherein said voltage-regulatingmeans comprises a voltage comparator for each sink capacitor and a firstswitch control means to prevent connection of its associated sinkcapacitor to said source capacitor so long as the voltage of said sinkcapacitor exceeds a predetermined level.
 7. A low voltage power supplycomprising a source capacitor, an AC power input, a phase-controlledrectifier to connect said AC input to said source capacitor during thedecreasing voltage part of the cycle of said AC power to charge saidsource capacitor to a relatively small part of the RMS voltage of saidpower input, a plurality of sink capacitors, first sequentially operatedswitches to connect said sink capacitors to said source capacitorseriatim, a second set of switches operated in the same sequence as saidsequentially operated switches but having a predetermined time delaytherefrom to connect said sink switches to an output load circuit, andoutput voltage control means to control opening of said firstsequentially operated switches.